This PR is a fix for issue #259

List of updates/fixes:
* module xlsx renamed to excel.
* Allow reading not only xlsx format but also xls, ods, xlsb
* Allow Excel DateTime format and transform it to arrow
Timestamp(Seconds, None)
* Allow using NULLs in any data types and use null value instead of
string "null"
* Fix issue with incorrect data type inference when multiple data types
are detected.
* Add possibility to specify data schema in config.
* Add new options: -
rows_range_start
 - rows_range_end
 - columns_range_start
 - columns_range_end
 - schema_inference_lines
* Make sheet_name optional and if it is not specified than use first
sheet by default

* Bump calamine crate to version 0.23.1 and add feature "dates"
(supporting for DateTime column format)

Documentation updates: roapi/docs#20